Oral prosthetic devices, in the form of dental implants are one class of biologically compatible components used in the body to replace injured or diseased body structures. Such components are biologically compatible in the sense that implantation of such devices has no adverse effect upon the living tissue, and the environment of the body does not adversely affect the device. It may be necessary to replace entire bony structures, fill bone defects, or replace extracted teeth.
The replacement of a bone section or a tooth offers the particularly challenging problem of finding a biologically compatible material which will allow tissue growth to penetrate into the material.
U.S. Pat. No. 4,186,448 to Brekke discloses a device and method for treating a newly created bone void or soft tissue deficiency. A body made of a biodegradable material, such as polylactic acid, including enclosed interconnected randomly positioned, randomly shaped and randomly sized voids extending throughout the mass of the body attracts blood by capillary action, leading to healing of the bone void as the body member biodegrades and is absorbed by the tissue. A wetting agent such as sodium oleate or Ultrawet 60L (T.M.) sold by the Fisher Scientific Company of Chicago, Ill. assists the capillary action (See Column 1, lines 55-62). This type of implant, while promoting healing and possibly preventing the bone structure around an extracted tooth from receding, does not provide a structure to which replacement crowns for extracted teeth can be attached.
"Fabrication and Characterization of Porous-rooted Polymethylmethacrylate (PMMA) Dental Implants," by Klawitter et al. published in the Journal of Dental Research, April 1977, p. 385 discloses polymethylmethacrylate dental implants having a porous coating on the root portions. The coating is developed by adding methylmethacrylate monomer to polymethylmethacrylate beads to soften the surface of the beads, and then compression molding the mixture at high pressure. Pore sizes of 20-40 micron diameter are indicated as being restrictive of bone growth but allowing for fibrous tissue ingrowth, while those of approximately 200 micron diameter are disclosed as appearing to be optimum for bone tissue ingrowth.
U.S. Pat. No. 4,199,864 to Ashman discloses a method for fabricating polymeric plastic implants for endosteal or periosteal applications having a porous surface with pores of a predetermined size, pore depth, and degree of porosity. Leachable substances, such as sodium chloride crystals of controlled particle size are added to a powdered polymer-liquid monomer mixture in relative amounts corresponding to the desired degree of porosity. These crystals, combined with mold release agents, are used to coat mold cavity surfaces to achieve proper near-surface porosity. After heat polymerization without the use of an initiator, and abrasive removal of resulting surface skin, the salt is removed by leaching to provide the desired porosity. Bone ingrowth is promoted by pore sizes in the 200-400 micron range. Pore sizes of 50-150 microns result in soft tissue ingrowth. The use of two different size pores in a single implant is disclosed at column 3, lines 29-36.